1. Field of the Invention
The present invention relates to right-angle board-mounted connectors and their printed circuit board (PCB) footprints. More specifically, the present invention relates to right-angle board-mounted connectors and PCB footprints with improved coaxial structure.
2. Description of the Related Art
It is known to use a board-mounted connector to connect a coaxial cable to a PCB. An example of such a known connecter 100 is shown in FIGS. 19-21. The connector 100 includes a housing 101 that mates with a corresponding connector of the coaxial cable (not shown) and a base 102 that can be soldered to PCB 105. The connector 100 also includes a center pin 104 that connects with a center conductor (not shown) of the coaxial cable and with a trace (not shown in FIGS. 19 and 20 but shown as trace 113 and 213 in FIGS. 22 and 24) on the PCB 105. The connection between the center conductor of the coaxial cable, the center pin 104 of the connector 100, and the trace of the PCB 105 allows signals to be transmitted between the PCB 105 and the coaxial cable. The connector 100 also include a dielectric 103 arranged in the bottom of the base 102 that surrounds the center pin 104. The center pin 104 has an L-shape with a first portion that extends along the central axis of the housing 101 and with a second portion that is perpendicular to the first portion and that extends parallel to the surface of the PCB 105. The shape of the dielectric 103 is toroidal with the center pin 104 extending through the center hole of the toroid and along a radial groove. That is, the dielectric 103 has a disk-like shape with a hole in the center and groove extending from the hole to the edge of the disk.
FIG. 26 shows another known connector 150. This connector 150 is similar to the connector 100 of FIG. 19, except that the connector 150 includes legs 156 that can be grounded when the connector 150 is mounted to PCB 160. Legs 156 are inserted into holes in the PCB 160 and are connected to ground in or on the PCB 160. Connector 150 includes a housing 151 that mates with a corresponding connector of a coaxial cable and a base 152 that can be soldered to PCB 160. The connector 150 also includes a center pin 154 that connects with a center conductor of the coaxial cable and with a trace on the PCB 160. The connector 150 also includes a dielectric 153 arranged in the bottom of the base 152 that surrounds the center pin 154. The center pin 154 has an L-shape with a first portion that extends along the central axis of the housing 151 and with a second portion that is perpendicular to the first portion and that extends parallel to the surface of the PCB 160. The shape of the dielectric 153 is toroidal with the center pin 154 extending through the center hole of the toroid and along a radial groove. That is, the dielectric 153 has a disk-like shape with a hole in the center and groove extending from the hole to the edge of the disk.
A coaxial cable has a center conductor that is surrounded by a dielectric that is surrounded by an outer shield. The shield provides a conductive surface that shields signals transmitted in the dielectric and defines the outer boundary of the transmission line. Transmitted signals in the dielectric couple from the center conductor to the shield's inner conductive surface. To maintain signal integrity, the connectors attempt to approximate or mirror the coaxial structure of the coaxial cable. However, the connector 100, 150 inexactly approximates this coaxial structure (i.e., a conductor that is surrounded by a dielectric that is surrounded by a shield). The housing 101, 151 of the connector 100, 150 is typically made of a conductive material and corresponds to the shield of the coaxial cable. The center pin 104, 154 is also typically made of a conductive material and corresponds to the center conductor of the coaxial cable. The dielectric 103, 153 is made of a dielectric material and corresponds to the dielectric in the coaxial cable.
High-speed signals act like guided waves. A uniform guiding system from beginning to end with no abrupt changes in geometry or impedance is ideal. One of the problems with the connectors 100, 150 is that the center pin 104 makes a 90° bend and then travels along the bottom surface of the base 102, which causes an abrupt change in the geometry and in the impedance. The signal transmitted by the portion of the center pin 104 extending along the printed circuited board couples with the closest conductive surface, which is the flat inside surface of the base 102, and thus the structure is no longer coaxial. When the connectors 100, 150 are mated with a coaxial cable, the signals transmitted between the connector 100, 150 and the coaxial cable experience this abrupt change in the coaxial structure.
Another of the problems with the connectors 100, 150 is that they provide only a two-dimensional interface between the connector 100, 150 and the PCB 105, 160 (the flat bottom surface of the connector 100, 150 that is soldered to the PCB 105, 160). When the connectors 100, 150 are mounted to a PCB 105, 160, the signals transmitted between the connector 100, 150 and the PCB 105, 160 experience an abrupt change in the coaxial structure because of this two-dimensional interface.
The legs 156 of the connector 150 do not help the connector 150 approximate the coaxial structure because the legs 156 are located too far away, even farther away from the center pin 154 than the shield is from the center conductor of a coaxial cable. That is, the legs 156 are located outside of the outer boundary of the transmission line defined by the footprint. Thus, the legs 156 have little effect other than mounting the connector 150 to the PCB 160 and providing ground continuity between the connector 150 and the PCB 160.
A coaxial cable can also have an outer sheath, but this is not typically mirrored in a connector.
Because the connector 100, 150 inexactly approximates the coaxial structure, signals transmitted through the PCB 105, 160 and the coaxial cable experience an impedance mismatch when the signals are transmitted through the connector 100, 150. This impedance mismatch decreases performance with higher frequencies. The larger the impedance mismatch, the worse the signal integrity, as shown in FIGS. 15-18.
FIGS. 22 and 23 show PCB 110 with a known footprint for the connector 100. The footprint of a printed circuit board refers to the layout of the structures on the PCB (e.g., ground planes, vias, traces, etc.) that are required to mount a component, which in FIGS. 22 and 23 is connector 100. The footprint is sometimes referred to as the break-out region (BOR). The footprint includes a base pad 112 and a pin pad 114. The base 102 is soldered to the base pad 112, and the center pin 104 is soldered to the pin pad 114. The pin pad 114 is connected to a trace 113 that leads away from the pin pad 114. The footprint also includes a ground plane 115 that surrounds trace 113.
FIG. 23 is a see-through perspective view of the footprint of FIG. 22 in which structures within the PCB 110 are shown using broken lines. These structures cannot normally be seen. The PCB 110 typically includes multiple layers. The layer nearest to the surface of the PCB 110 includes a ground plane 116. The ground plane 116 includes an antipad 117 underneath the pin pad 114. The ground plane 116 is connected to the ground plane 115 on the top of the PCB 110 by vias 118. The reference lines for ground plane 116 and antipad 117 are broken to show that the ground plane 116 and the antipad 117 are within the PCB 110.
FIGS. 24 and 25 show PCB 210 with another known footprint for the connector 100. This footprint is similar to the footprint of FIGS. 22 and 23, except that base pad 112 of FIGS. 22 and 23 and the base pad 212 of FIGS. 24 and 25 have different interior shapes. The base 102 is soldered to the base pad 212, and the center pin 104 is soldered to the pin pad 214. The pin pad 214 is connected to a trace 213 that leads away from the pin pad 214. The footprint also includes a ground plane 215 that surrounds trace 213.
FIG. 25 is a see-through perspective view of the footprint of FIG. 22 in which structures within the PCB 210 are shown using broken lines. The PCB 210 typically includes multiple layers. The layer nearest to the surface of the PCB 210 includes a ground plane 216. The ground plane 216 includes an antipad 217 underneath the pin pad 214. The ground plane 216 is connected to the ground plane 215 on the top of the PCB 110 210 by vias 218. The reference lines for ground plane 216 and antipad 217 are broken to show that the ground plane 216 and the antipad 217 are within the PCB 210.
The footprints of FIGS. 21-24 also inexactly approximate the coaxial structure of a conductor that is surrounded by a dielectric that is surrounded by a shield. The traces 113, 213 are surrounded on the sides by the ground planes 115, 215 on the top of the PCB 110, 210 and the vias 118, 218 connecting the ground planes 115, 215 and 116, 216. The traces 113, 213 are surrounded on the bottom by the ground planes 116, 216 on the layer within the PCB 115, 215 nearest the surface.
Known techniques to improve signal integrity focus on providing the best possible uniform wave guiding system, which is sometimes referred to as the transmission line. A fundamental parameter used to define a uniform guiding system is the characteristic impedance (Zo). Today's high-speed data transmission is high frequency. This allows characteristic impedance Zo to be defined as the square root of the ratio of inductance L to capacitance C (i.e., Zo=√(L/C)). The inductance and capacitance values are determined by the material properties and geometrical dimensions of the finite length of section of the guiding system. The known technique of impedance matching uses the material properties and geometrical dimensions of different sections of the wave guiding system to provide capacitance and inductance changing schemes to attempt to achieve the most overall uniform wave guiding system from start to finish. Such is the case for increasing the number of vias 118, 218 in PCB 110, 210 and changing the shape of antipads 111, 117, 211, 217.
Impedance matching for wave guide systems encompass many components, such as PCB traces, dielectric material, vias, antipads, interfaces, connectors, and interconnections. Known impedance matching techniques include [1] via optimization, [2] antipad optimization, [3] backdrilling techniques, [4] inductive compensation using curved PCB traces, and [5] new dielectric material laminates.
However, these known techniques for improving signal integrity in the PCB have limited ability to improve signal-integrity performance, particularly as the frequency of the signals increase. Ideally, the signals should propagate in the transverse electromagnetic (TEM) mode, with minimal loss and reflections.
FIGS. 15-18 compare the electrical characteristics of assembly of the connector 100 and the known footprint of FIG. 22 with the assemblies of the connectors and footprints of various preferred embodiments of the present invention. As shown in FIGS. 15 and 16, the insertion loss and the return loss are frequency dependent. The lower insertion loss is a combination of reflection losses, real losses, and non-TEM mode, causing a loss in transmission. FIGS. 17 and 18 show the near-end crosstalk (NEXT) and far-end crosstalk (FEXT) coupling of adjacent coaxial cable assemblies including a coaxial cable, a connector, and a footprint.
The known connector and footprint are typically designed to be used together to support high-speed data signals; however, improving one without improving the other has limited success in improving the overall signal integrity.